1. Field of the Invention
The present invention relates to a holder for a drive piston of a setting tool.
2. Description of the Prior Art
European Publication EP-O 346275 B1 discloses an explosive powder charge-operated setting tool including a piston guide and a drive piston displaceable in the piston guide. The piston guide has radial openings facing the drive piston, and spring-biased braking balls extending through the radial openings and engaging the drive piston. The spring, which applies a biasing force to the braking balls is formed as a ring spring for applying a radially acting, with respect to the piston, biasing force to the braking balls. The ring spring is provided on its inner profile with a bearing surface acting on the braking ball. The bearing surface is inclined to the piston at an acute angle that opens in a direction opposite a setting direction.
In the ignition-ready position of the drive piston, the braking balls, which are supported against the ring spring, engage the outer surface of the drive piston.
When the drive piston moves in the setting direction, it entrains the braking balls therewith. The braking balls expand the ring spring, which results in the bearing surface transmitting the radial biasing force to the braking balls. In this way, the braking balls are pressed radially against the piston body by the ring spring, braking the same. Even with a small displacement of the drive piston in a direction opposite the setting direction, the braking effect can be substantially reduced or eliminated, as the braking balls displace in the same direction as the drive piston, unloading the ring spring. After being unloaded, the ring spring does not press any more the braking balls against the piston body.
The piston holder according to EPO 346 275 B1 has a rather complicated structure that includes a plurality of a braking balls, a braking ball-biasing ring spring and further springs that bias respective braking balls in a direction to the ring spring.
An object of the present invention is to provide a piston holder having a simplified design and which would reliably retain the drive piston in its ignition-ready position in the absence of ignition.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a piston holder for a drive piston of a setting tool that includes a circumferential groove provided in a stationary, with respect to the setting tool, component of the setting tool and surrounding the drive piston. The circumferential groove becomes shallower in a drive-out direction of the drive piston. The piston holder further includes an O-shaped helical tension spring located in the circumferential groove and concentrically surrounding the drive piston.
The O-shaped helical tension spring engages the piston body of the drive piston, applying a rather small bearing force and little friction. Because the O-shaped helical tension spring extends in the circumferential direction of the groove or the piston body, it is relatively long and, therefore, has a small spring rate. The small spring rate results in a small bearing force applied to the drive piston. The small torus diameter lies in the widened region of the groove which becomes shallower in the drive-out direction of the drive piston. The groove opens toward the guide channel, in which the drive piston is displaced, and has a bottom remote from the drive piston and which approaches the drive piston in the drive-out direction of the drive piston. When the drive piston is displaced in its drive-out direction, without the setting tool being ignited, it entrains the O-shaped helical tension spring therewith and the friction force between the spring and the drive piston increases, with the spring being displaced into the shallower portion of the groove. The increase of the friction force is caused by a radial deformation of the spring in the narrower of shallower section of the groove. When the force that pushes the drive piston in its drive-out direction is eliminated, the helical tension spring rolls back under the action of its spring force. Upon rolling back, the spring entrains the drive piston back, at least to some extent, returning the drive piston in its ignition-ready position.
Upon ignition of the setting tool, with the increase of the displacing force, the friction force between the helical tension spring and the drive piston is overcome, with the drive piston being able to drive a fastening element into a constructional component. Generally, the spring characteristics limit the friction forces in such a way that no section of the spring breaks. The friction forces are retained in an anticipated range. The friction forces are used for braking the drive piston. The O-shaped helical tension spring does not hinder return of the drive piston to its initial, ignition-ready position, as the friction between the spring and the drive piston becomes sharply reduced as the drive piston returns to its initial piston, with the spring being displaced in the deeper region of the groove where it does not apply any noticeable pressure to the drive piston.
The drive holder according to the present invention is easy to produce and is easy to mount. Therefore, it is very economical. Moreover, it is substantially maintenance-free.
According to the present invention, in order to form the O-shaped, helical tension spring, opposite ends of a straight section of a helical tension spring are screwed into each other, upon bending the straight section. By selecting the screw-in depth, the friction force between the helical tension spring and the drive piston can be adjusted.
For forming the O-shaped helical tension spring, two straight spring sections can be used, with the opposite ends of one section being screwed in opposite ends of the other spring.
The helical tension spring according to the present invention is more stiff in the screw-in region than in other regions of the spring. This results in that the forces imparted by the spring to the drive piston are not symmetrical. The drive piston is pressed radially against the guide channel, which results in generation of additional friction forces which adversely affect the return movement of the drive piston.
This drawback is eliminated by forming the O-shaped helical tension spring of two straight sections having the same length. With the formation of the O-shaped spring of two sections, two screw-in regions are offset relative to each other by 180xc2x0 in the circumferential direction. Thereby, the unsymmetrical application of spring forces to the drive piston is eliminated.
With two screw-in location, the screwing is effected with the helical tension spring sections being twisted in opposite directions. With this, the spring ends of the two spring sections are held together, while the tension is reduced. With the opposite ends of the two spring sections being screwed into each other, they do not become loose by themselves, as the resiliency of the spring prevents unscrewing.
In order to increase the service life of the helical tension spring, according to the present invention, it is formed of a spring wire having a rectangular cross-section or of a stranded wire. In the later case, more wear material is available. On the other hand, with the helical tension spring being formed of a strand material, a certain redundancy is obtained. This means that the spring does not break rapidly when a strand is sheared off or is broken.
Further, the service life of a helical tension spring is increased when it is provided with a coating. As a coating, e.g., TiN-coating, TiC-coating, or a coating of a diamond-like carbon can be used. The coating, which is formed of one of the above-mentioned material or a material having similar characteristics, is relatively hard, which increases the stability of the spring. The foregoing coatings can be formed with the use of vacuum metallization, which permits to form them at a relative cold temperature. This prevents the helical tension spring from being damaged as a result of heat treatment.
Alternatively, the drive piston body itself can be decarburized. This not only increases the service life of the drive piston itself but also permits to make the drive piston body less hard than the spring. This also increases the service life of the spring.
The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages an objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings.